handling of the crop, but in general it is a fact that the whole sub- 

 ject of the cause and prevention of the rotting of greenhouse lettuce 

 is not at all understood, either by practical growers or vegetable 

 pathologists. Various diseases of lettuce other than rotting are 

 known, but they are seldom or never troublesome to experienced 

 growers in this section. The most trouble is caused by certain 

 forms of rotting, which may now be described. 



"Damping Off" or Rotting of Seedlings. 



All lettuce growers and gardeners generally are familiar with the 

 dying of seedlings, prickers, and cuttings by what is known as 

 " Damping Off." This is not often destructive to lettuce, owing to 

 the low temperature at which the plants are grown, although in 

 almost any lot of seedlings some affected plants may be found. 

 Damping is favored by excessive moisture, high temperature, and 

 stagnant air ; i. e. poor ventilation, and affects especially plants 

 which are very close together. All good lettuce growers understand 

 this and guard against the trouble by avoiding these conditions. 

 Affected seedlings rot off in the stem and soon wither away. The 

 cause of this disease is not very definite, as the same effect is pro- 

 duced by several different fungi, which get a start in the stem of the 

 plant, and, growing there, cause it to rot. One fungus which often 

 causes it is the common dusty gray mould which is seen in any 

 greenhouse upon dead flowers, leaves, etc., and this is of particular 

 importance, not from the injury which it causes by Damping but 

 from its further relation to the lettuce plant. This mould, which is 

 called Botrytis, may usually be seen on some of the plants in a lot 

 of lettuce seedlings, covering the decaying, withered remains with a 

 gray, dusty growth, or not rarely it appears only on leaves, either 

 dead or living, while the plant is still alive. (See Fig. i.) The 



death of the plant is brought 

 about by the fine, thread- 

 like branches of the fungus 

 which grow all through the 

 stem and completely demor- 

 alize its tissues. When it 

 gets on to the leaves they 

 _ ^ , , , , , are killed in the same way. 



Fig. I. Growth of Botrytis, considerablyenlarged, . '' 



showing the upright branches bearing the spores. but it may nOt spread tO 



